The present invention relates in general to the field of detection, and more particularly to a structure and method for protecting integrated circuit sensors from the environment of intended use.
Without limiting the scope of the invention, its background is described in connection with the protection of integrated circuit fingerprint sensors from the environment during regular use, as an example.
Heretofore, in this field, the detection of fingerprint patterns has been useful for the identification of specific individuals based on the observation that each individual person has a unique fingerprint. Fingerprints, therefore, can be used not only to positively identify individuals, but to exclude individuals whose fingerprint profile does not match a pre-existing set of patterns.
Fingerprint sensing has evolved from optical and mechanical sensing technology that acquires a fingerprint image. In those systems generally, the mechanical and optical sensors obtained a fingerprint image using a scanner or a camera, processed the acquired information into an analog or digital signal that could be analyzed, and provided an output based on the acquired signal. Unfortunately, the lighting and contrast conditions available at the time the image was acquired affected the analysis and consequently the output from the sensor.
Another class of fingerprint sensors are capacitive sensors, such as that disclosed in U.S. Pat. No. 4,353,056 issued to Tsikos. The Tsikos patent demonstrates the use of a sensor that incorporates a sensing member that has a sensing surface for receiving a fingerprint. The sensing surface has a means for sensing the ridges and valleys of the skin of the finger under observation. A sensing member contains a multitude of capacitors that sense the patterns of the fingerprint when the finger is pressed against the sensing surface. The information obtained is transformed into an electric signal. The capacitors are insulated from the environment of use by a flexible membrane that conforms itself to the contour of the fingerprint. Unfortunately, the repeated cycles of flexing and compression of the flexible membrane can lead to device failure and the need to replace the membrane.
U.S. Pat. No. 4,428,670 issued to Ruell, et al., discloses a fingerprint sensor that provides an electric output signal in response to the topographic relief of the fingerprint. The sensor incorporates a contact body that is formed at least in part by a light transparent elastic material. The elastic contact material can be attached to a flat sensor plate that has a light receiving surface. The sensor also incorporates a light source and a photodetector to measure the valleys and ridges of the fingerprint. The elastic nature of the contact body, of the above described sensor causes cycles of compression and flexing that lead to the deterioration of the contact point between the sensor and the finger.
It has been found, however, that the present methods and structures for protecting fingerprint sensors from the environment of intended use fails to address the distinct environmental exposures to which the sensors are exposed. For example, under ideal conditions the user would gently place the finger on the plate without an excess of pressure or shock. Unfortunately, it is the case that the sensor surface will be exposed to a wide variety of pressures, and that objects other than fingers might come in contact with the sensor surface.
Another problem with current sensors is the need to protect the sensor from electrostatic discharges, e.g., static electricity, caused by the user and the sensor being at different voltage potentials. Users can be exposed to environmental conditions that cause a great difference in potential in comparison to objects that are at a different potential or ground. Such a difference can be caused by users shuffling their feet across a carpet. When the user approaches the sensor at a great voltage disparity, a sudden electric discharge may cause operational failure of the sensor, both temporary and permanent. Although the current flowing from the discharge may be small, damage to the sensor or the data flowing from the sensor can still occur. While damage to the data or the sensor should be avoided, the sensitivity of the sensor should be maintained at close to optimal levels.
Yet another significant problem of current structures for the protection of fingerprint sensors is contamination from substances, such as oils and proteins that are found on the surface of fingers. To remove these contaminants, it is often necessary to use organic or inorganic solvents or detergents to clean the sensor surface.
Another area of concern is hygiene. Fingers, as well as the environment, tend to contain a number of microbes that need to be removed from the sensor along with finger contaminants. To remove these microbes and reduce the chance of passing a contagion between users, antibacterial, antifungal and decontaminating agents are used to clean the sensors. These decontaminating agents often include harsh abrasives, enzymes, organic or inorganic solvents, or detergents. Furthermore, the sensors are often exposed to oxidating environments, UV rays, and the like during normal use.
What is needed is a structure and method to protect fingerprint sensors from electrostatic discharges, while at the same time maintaining the sensors ability to withstand mechanical stress. The structure should permit continued functioning of the sensor during normal use, and be able to withstand, among other things, the extreme conditions of humidity, electricity, heat, light, etc., to which the sensor may be exposed. The sensor structure should also be resistant to chemical detergents and solvents, but be compatible with the underlying components of the sensor.
In one embodiment, the invention is directed to an integrated circuit sensor comprising an integrated circuit containing areas of sensing circuitry over which an insulating layer is disposed. The insulating layer helps to electrically isolate the sensing circuitry from subsequent layers and the environment. Next, a discharge layer is formed that is electrically or semi-electrically conductive. The discharge layer dissipates electrical discharges that may be caused when a user touches the sensor. The discharge layer is doped with a dopant in areas disposed over the sensing circuitry and may or may not be doped in other areas. A mechanical protection layer may also be disposed over the discharge layer to provide hermetic and mechanical protection for the underlying circuit. The discharge layer may comprise a silicon-based layer that is partially doped over the entire sensor to increase conductivity. Additionally, the discharge layer may be more heavily doped in areas away from sensing areas. The additional doped areas are coupled to a chip ground that may be coupled to a system ground. In one embodiment the discharge layer is an SiCx layer, where x is less than 1.
The sensitivity of an integrated circuit sensor can be degraded by adding a highly conductive layer, such as a metal layer. Likewise, it is herein recognized that the discharge layer or sensor surface should be resistant to mechanical stress caused by environmental conditions and use, e.g., scratches. Therefore, the discharge layer is electrically isolated from the functional components of the sensor, for example, capacitors, by including an insulating layer. In one embodiment, the sensor is protected from electrostatic discharges by a passivation that can integrally contain one or more semi-electrically conductive layers, with at least one layer being a discharge layer. Preferably, the discharge layer is an SiCx layer, where x is varied to maximize hardness and optimize conductivity. In another preferred embodiment the discharge layer is a silicon-based layer that is doped to optimally carry electrical discharges, while at the same time maintaining sensor device sensitivity. The discharge layer can be uniformly doped with a charge-carrying dopant to increase its ability to keep an electric discharge away from the sensor circuitry. Alternatively, the discharge layer may have non-uniform doping where more dopant is included in the discharge layer above areas that do not cover sensing circuitry. In this way, the discharge layer can protect the sensing circuitry from a higher level of electrostatic discharge than an undoped or uniformly doped discharge layer, while maintaining the resolution of the sensor circuitry. The mechanical protection layer and the discharge layer should be compatible and, in one embodiment, can be concurrently formed on the insulating layer. Concurrent deposition of the mechanical protection layer and the discharge layer can be accomplished by, for example, beginning the deposition with SiC and thereafter decreasing the amount of carbon to make a SiCx layer, where x is less than 1.
In another embodiment, the insulating layer of the integrated circuit passivation can be a silicon oxide layer. Alternatively, the first insulating layer can be silicon nitride. The integrated circuit passivation may further comprise a second insulating layer disposed between the first insulating layer and the mechanical protection layer. The second insulating layer can be silicon oxide or silicon nitride. In yet another embodiment the insulating layer is silicon oxide, and the second insulating layer is silicon nitride, with the discharge layer whose hardness is greater than that of silicon nitride being partially conductive. A mechanical protection layer can also be disposed above or below the discharge layer.
Yet another embodiment of the present invention is a method of fabricating a fingerprint sensor passivation comprising the steps of, obtaining an integrated circuit having areas of sensing circuitry separated by offset areas, depositing a first insulating layer on the integrated circuit and depositing over the first insulating layer a discharge layer that is semi-electrically conductive. The method may further comprising the step of depositing a second insulating layer between the first insulating layer and the discharge layer, and may also include depositing a mechanical protection layer on or below the discharge layer. The mechanical protection layer and the discharge layer should be compatible and, in one embodiment, can be concurrently formed on the insulating layer. Concurrent deposition of the mechanical protection layer and the discharge layer may be accomplished by, for example, beginning the deposition with SiC and thereafter decreasing the amount of carbon to make a SiCx layer, where x is less than 1. The discharge layer can have dopant implanted in areas above the offset areas. Additionally, dopant can be implanted in the entire discharge layer.